Configuring OSPF

Designated Router Election

In a network with no designated router and no backup designated router, the neighboring router with the highest priority is elected as the DR, and the router with the next largest priority is elected as the BDR, as shown in Figure 8.2

Designated Backup Router

 

 

 

 

 

 

 

 

 

priority 10

Router A

 

 

 

 

 

 

 

 

 

 

Designated Router

priority 5

priority 20

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Router C

Router B

Figure 8.2 Designated and backup router election

If the DR goes off-line, the BDR automatically becomes the DR. The router with the next highest priority becomes the new BDR. This process is shown in Figure 8.3.

NOTE: Priority is a configurable option at the interface level. You can use this parameter to help bias one routing switch as the DR.

Designated Router

 

 

 

 

 

 

 

 

 

 

 

priority 10

Router A

Designated

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

X

Backup

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Router

priority 5

priority 20

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Router C

Router B

Figure 8.3 Backup designated router becomes designated router

If two neighbors share the same priority, the router with the highest router ID is designated as the DR. The router with the next highest router ID is designated as the BDR.

NOTE: By default, the HP router ID is the IP address configured on the lowest numbered loopback interface. If the routing switch does not have a loopback interface, the default router ID is the lowest numbered IP address configured on the device. For more information or to change the router ID, see “Changing the Router ID” on page 6-25.

When multiple routing switches on the same network are declaring themselves as DRs, then both priority and router ID are used to select the designated router and backup designated routers.

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